Abstract. There is a critical role for the thymus in certain virus-induced leukemias. In vitro analysis of the phenomenon to define the role of thymic epithelial cells has produced conflicting results, probably because of the use of cultures of mixed cell types considered to be "thymic epithelium". We have adapted methodology, used successfully to culture epidermal cells, to the culture of thymic epithelium in virtually pure form as assessed by immunofluorescence staining for keratin. Using such cultures, we propose to investigate the virogenic status of thymic epithelial cells from various mouse strains for ecotropic, xenotropic, and mink cell focus-inducing (MCF) murine leukemia viruses (MuLVs). The susceptibility of thymic epithelial cells to exogenous infection will also be examined. Virus produced by thymic epithelial cells will be assayed for thymotropic and transforming characteristics. Finally, we will examine the thymic epithelial cell for its capacity to act as a unique site of recombination between ecotropic and xenotropic viruses to produce MCF virus, the latter now considered the proximate agent in thymic viral leukmogenesis in the AKR mouse. Through these experiments we hope to clarify the role of thymic epithelium in this process.